Multi-phase clock signal generation circuits are widely used in integrated circuits, and serve as important components of multi-phase charge pumps. Conventionally, a multi-phase clock signal is generated by a combination of delayed clock signals. In such conventional multi-phase clock generator, characteristics of elements in the clock generator, such as MOS transistor, resistor, may vary significantly with changes in integration process, operating voltage or operating temperature. Clock phases which depend on these elements may be accordingly advanced or delayed, and some of the phases may even overlap with each other. As shown in FIG. 7, the solid lines denote normal waveforms of clock signals CLK1, CLK2, and the dashed lines denote potential changes in the waveforms of clock signals CLK1, CLK2 with fluctuations in integration process, operating voltage or operating temperature. As can be seen from FIG. 7, the phases of the clock signals overlap with each other. In the case that the conventional multi-phase clock generator is used in a multi-phase charge pump, the efficiency of the multi-phase charge pump would be considerably reduced when there is fluctuation in integration process, operating voltage or operating temperature.